Question: Which of these numbers is prime? ${10,\ 17,\ 25,\ 49,\ 63}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 10 are 1, 2, 5, and 10. The factors of 17 are 1 and 17. The factors of 25 are 1, 5, and 25. The factors of 49 are 1, 7, and 49. The factors of 63 are 1, 3, 7, 9, 21, and 63. Thus, 17 is a prime number.